Mass balance modeling highlights the role of the littoral zone in modulating the cycling of phosphorus in a large, multi-basin lake (Lake Erie)

Excess land-derived input of phosphorus (P) is generally considered the primary driver of eutrophication symptoms in large lakes, including Lake Erie, one of the Laurentian Great Lakes. Here, we present a regionalized P mass balance model for Lake Erie’s three basins (western, central, and eastern) that explicitly accounts for the decadal circulation of the lake and P fluxes into and out of the lake’s nearshore (littoral) zones. The model comprises 11 water column compartments, including both nearshore and offshore compartments, as well as a further partitioning of the offshore waters between surface and deep-water compartments. External P loads to the lake, plus water fluxes into and out of the different lake compartments were averaged over the 2003–2016 period and imposed to the model. According to the steady state baseline simulations, 85% of total P (TP) flowing out of the western basin of Lake Erie enters the lake’s central basin along the nearshore zone. Moreover, transport parallel to the coastline, both on the northern (Canada) and southern (USA) sides of the central basin, represents the major pathway for the continued eastward movement of P to the eastern basin and, ultimately, to the outflow discharging in downstream Lake Ontario. The results further underscore (1) the importance of shoreline erosion as a TP input to the littoral zone, especially along the Canadian side of Lake Erie’s central basin, (2) the complex, bi-directional P exchanges between nearshore and offshore waters in the central and eastern basins, and (3) the sensitivity of these exchanges to the lake’s general circulation regime.